When producer David Foster wanted to blend Rod Stewart’s vocals with those of the late Ella Fitzgerald on the classic “What are you doing New Year’s Eve?” he relied on a sort of secret sauce to get the cleanest tracks possible — Audionamix’s ADX technology.

Already in use for everything from film and television to live performances at the Hollywood Bowl, ADX made it possible to do a clean extraction of Fitzgerald’s voice from an old recording of a beloved holiday song and then mix that track with a new recording of Stewart’s voice and updated music for the artist’s “Merry Christmas, Baby” CD.

“When David worked with the Audionamix technology he said it created a track so clean that it defied logic,” says Jay Landers, senior VP of A&R for Verve Music Group, who regularly works with Foster. “It’s really opened doors for us in terms of what’s possible.”

Foster’s track record with these types of projects includes producing the duet of “Unforgettable,” on which Nat King Cole’s voice was paired with his daughter Natalie Cole after the elder Cole was deceased.

ADX’s technology was originally developed for the film “La Vie en Rose.” Helmer Olivier Dahan wanted independent music stems so that a surround sound mix could be made for exhibition. Since the old Edith Piaf recordings were either in mono or stereo, there weren’t any available stems that would do the trick. Audionamix used a proprietary technology for the stem creation and Dahan was able to adjust the position of Piaf’s voice as well as the musical instrumentation to fit the camera angles he used.

“We’re always pushed by our clients and their requests,” says Arnaud Dudemaine, VP of operations and business development for Audionamix. “When they come to us with a need we often have to develop entirely new solutions for them.”

Hans Zimmer approached Audionamix for vocal and instrumental track extractions for “Inception.” Projects like the re-releases of “Psycho” and “The Blues Brothers” followed. They also managed the vocal and instrumental isolation on the original “West Side Story” so that the film could be screened with the L.A. Philharmonic performing music while the classic dialogue and original singing could still be heard with a live orchestra.

They’ve also done music dissociation — taking the licensed and often costly music from a composite mix while still keeping the dialogue and effects intact — for DVD and foreign market releases of shows like “Melrose Place,” “Beverly Hills, 90201,” and “Cheers.”

“As new ways of using content come up, there are more ways to use ADX,” says Rick Silva, Audionamix VP of production. “We can’t really anticipate how this technology will be used next because it’s our customers who are going to tell us what they need based on what they’re trying to create.”

Both Silva and Dudemaine say there are more revolutionary uses of ADX to come in 2013, but neither can talk about the projects just yet.

“Many artists dream of making great artists of the past reappear,” Dudemaine says. “It’s a great pleasure to be part of that happening.”